1. Field of the Invention
The present invention relates to a disk-shaped magnetic recording medium composed of a non-magnetic support and a magnetic layer which, formed on the support, contains a magnetic powder and a binder for it. The invention further relates to a tape-shaped magnetic recording medium composed of a non-magnetic support, a magnetic layer which, formed on a main part of one side of the support, comprises a magnetic powder and a binder for it, and a back coating layer which, formed on a main part of the other side of the support, comprises carbon black dispersed in a binder. Particularly, the invention is directed to disk-shaped or tape-shaped magnetic recording media superior in electro-magnetic conversion characteristics and in durability.
2. Description of the Prior Art
Various disk-shaped magnetic recording media including so-called floppy disks and hard disks have been put to practical use and in the field of these recording media it is advanced year after year to make the density of recording higher and the wavelength of recording signals shorter. This is also true in the field of tape-shaped magnetic recording media for use in audio and video equipment, computers, etc. In response to this, it has been contemplated, in respect to magnetic recording media of the coating type comprising a non-magnetic support overlaid with a magnetic layer which contains a magnetic powder and a binder, that the magnetic powder to be dispersed in the magnetic layer is made finer and packed more densely. As a result, the surfaces of such magnetic layers have become much smoother and electro-magnetic conversion characteristics, including output characteristics, of those recording media have been improved; that is widely known.
However, these surface-smoothed magnetic recording media, as compared with conventional ones, tend to be low in traveling endurance or in the abrasion resistance magnetic coating layers, because the magnetic layer-magnetic head contact area as well as the coefficient of friction of magnetic coating layer increase with the increasing smoothness of this layer. That is to say, the contact of the magnetic layer with the magnetic head causes magnetic particles to fall off from the magnetic layer and these magnetic particles stick on the magnetic head, blinding the gap of the head. This tends to bring about great difficulties or serious troubles in recording and in reproduction.
Measures against this are already proposed for the purpose of fortifying such magnetic coating layers and achieving proper cleaning effect on magnetic heads. The measures comprise incorporating a relatively hard non-magnetic powder, e.g. an Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, or .alpha.-Fe.sub.2 O.sub.3 powder, as an abrasive into the magnetic layer and, if necessary, using a solid or liquid lubricant jointly with the non-magnetic powder, as disclosed in documents, e.g. Japanese Patent Publication Nos. Sho. 57-49967 and Sho. 57-49969 and Japanese Patent Application Kokai Nos. Sho. 60-5421, Sho. 60-7614, and Sho. 61-73240.
Examples of U.S. Patents disclosing abrasives for magnetic tapes are as follows:
U.S. Pat. No. 4,275,115 (Jun. 23, 1981) , two abrasives of at least 7 and less than 7 in Mohs hardness. PA1 U.S. Pat. No. 4,399,189 (Aug. 16, 1983) , (Al.sub.2 O.sub.3 and/or Cr.sub.2 O.sub.3)+(TiO.sub.2, . . . and/or CeO.sub.2). PA1 U.S. Pat. No. 4,423,452 (Dec. 27, 1983), TiO.sub.2 (&gt;0.4.mu.)+other abrasives. PA1 U.S. Pat. No. 4,460,651 (Jul. 17, 1984), .alpha.-Al.sub.2 O.sub.3 (.ltoreq.1.mu.) (.alpha.-form content 60-90%). PA1 U.S. Pat. No. 4,618,536 (Oct. 21, 1986), .alpha.-Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3. PA1 U.S. Pat. No. 4,748,081 (May 31, 1988), specific .alpha.-Fe.sub.2 O.sub.3. PA1 U.S. Pat. No. 4,756,962 (Jul. 12, 1988), .alpha.-Al.sub.2 O.sub.3 (0.1-0.3.mu.) (.alpha.-form content .gtoreq.50%).
For reducing the coefficient of friction of tape-shaped magnetic recording media during travel and thereby improving the durability, there is already proposed a measure which comprises providing a back coating layer (hereinafter abbreviated as a back layer) containing a filler dispersed in a binder, on the side opposite to the magnetic layer side of a non-magnetic support. In many cases, such back layers are formed with the objects of improving magnetic recording media in antistatic properties and providing light shields, besides the above-mentioned objects, and carbon black is used as a filler in these back layers. The object of improving in antistatic properties is to inhibit electric charge generation on magnetic recording media and thereby lessen the adhesion of dust or dirt, which are responsible for "dropout". The object of providing light shields is to reduce the malfunction of a light sensor which is used when the initial end and final end of a magnetic recording tape can be detected with a change in the light transmittance of the tape.
For improving the abrasion resistance of the back layer containing carbon black, the same measures as in the case of the magnetic layer are already proposed that comprise incorporating a relatively hard non-magnetic powder, e.g. an Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, or .alpha.-Fe.sub.2 O.sub.3 powder, as an abrasive into back layers and, if necessary, using a solid or liquid lubricant additionally. An example of these measures: U.S. Pat. No. 4,741,954 (May 3, 1988), carbon black+non-magnetic powder (Mohs hardness .gtoreq.5).
However, the above conventional abrasives incorporated into magnetic layers or back layers are generally in granular form and it is necessary for the coat reinforcement to use either larger granules of abrasive or a large amount of smaller granules of abrasive. In any case, requirements for magnetic recording media in respect to electro-magnetic conversion characteristics are difficult to satisfy with conventional granular abrasives. That is, low electro-magnetic conversion characteristics are caused by the low surface smoothness of magnetic layers when a large granular abrasive is used, and by the low magnetic powder content in magnetic layers when a smaller granular abrasive is used in a large amount. In the case of back layers, the surface smoothness thereof lowers as the particle size and amount of abrasive are increased. As a result, projections or depressions at the back layer surface, when the magnetic tape is wound up into a roll, are reproduced at the magnetic layer surface. Such phenomena are responsible for lowing in the electro-magnetic conversion characteristics of magnetic recording media.
Examples of the abrasives having shapes other than granular shapes are disclosed in Japanese Patent Application Kokai Nos. Sho. 57-162127 (acicular .alpha.-Fe.sub.2 O.sub.3), Sho. 58-68232 (fine flat powder), Sho. 61-220127 (acicular inorganic powder of more than 5 Mohs hardness), Sho. 61-283030 (granular Al.sub.2 O.sub.3 +angular Al.sub.2 O.sub.3 particles), Sho. 62-92128 (acicular abrasive), and Sho. 62-219327 (powders of an oriented shape, plate shape, and needle shape are described as examples). The effect and problem of the addition of these abrasives are nearly the same as in the case of the above stated granular abrasives.
For reasons as stated above, it is strongly desired today to further improve magnetic recording media in electro-magnetic conversion characteristics and in durability at the same time.